The development of new light materials with high mechanical performance has made it possible to produce prosthetic feet, which to a considerably greater extent than previously; liken the natural foot with regard to movement pattern while at the same time enabling the operative force to be stored and regained with decreased loss of energy.
While earlier prosthetic feet were either relatively stiff or were flexible to absorb energy, they limited the user to relatively slow and less strenuous activities, such as walking etc. Prosthetic feet have more recently been produced having an elastic core which stores energy when the foot is put on a substructure (heel strike), and gives up this energy as the foot is thrust away from the substructure (toe-off), thus simulating the active plantar flexion of the natural foot. This enables more demanding activities such as tennis, running etc.
An example of this kind of foot is described in the U.S. Pat. No. 4,547,913 and is sold under the trade name Flex-Foot by Flex-Foot Inc., Irvine, Calif. USA; This foot is made from carbon fibre composite material including a leg portion, a foot portion and a heel portion, all of which are stiffly connected to each other, and all three being spring elements with considerable elastic flexibility which enables the release of absorbed energy and permits the amputee to participate in sports activities, e.g. running or tennis. In a commercial embodiment, the foot portion is integrated with the leg portion and extends downwards and forwards from the ankle region, while the heel portion is a member attached to the lower part of the leg portion such as to extend downwards-backwards from the ankle region. In another commercial variant, the heel portion is substantially flat and extends backwards from approximately the middle part of the foot portion to which it is attached.
A prosthetic foot which is based on similar principles is the so-called SEATTLE foot (U.S. Pat. No. 4,645,509) where the lower leg part is fastened to an energy storing spring member with the shape of approximately a U placed horizontally with the opening forwards. Here lower leg part is fastened to the upper somewhat shorter leg of the U with the lower leg of the U fixed in the foot cosmesis.
A somewhat different type of prosthesis with an energy storing core is described in British Patent No. 2 187 102. The prosthetic foot described here has a first spring member extending from the heel to the forward part of the foot, and which is intended to store and release energy on being loaded by normal body movements. For taking up larger loads, there is arranged a second spring member above, and at some distance from the first one, so that when there is sufficient load the first member is bent into engagement against the second member, subsequent to which both spring members are deflected together.
A disadvantage with the above described energy storing and energy releasing prosthetic foot types is, inter alia, that the foot must be adjusted to the individual patient both in respect of this person's weight and the desired activity which the foot shall perform. This means that a prosthesis intended for use in relatively demanding activities, e.g. running or tennis, will not be as comfortable for normal activities such as normal ambulation, due to the required spring stiffness, and vice versa.
In addition, these known prosthetic feet do not provide any damping of the return movement of the foot, resulting in the risk of resonance phenomena. Further disadvantages include defective absorption of torsional loading, i.e. turning the foot, with subsequent risk of sliding between the prosthesis sleeve and the amputation stump; and relatively heavy deformation of the foot sole is when the foot is deflected.